“Coiled tubing injectors” are machines for running pipe into and out of well bores. Typically, the pipe is continuous but it can also be jointed pipe. Continuous pipe is generally referred to as coiled tubing since it is coiled onto a large reel when it is not in a well bore. The terms “tubing” and “pipe” are, when not modified by “continuous,” “coiled” or “jointed,” synonymous and encompass both continuous pipe, or coiled tubing, and jointed pipe. “Coiled tubing injector” refers to machines used for running any of these types of pipes or tubing. The name of the machine derives from the fact that it is was originally used for coiled tubing and that, in preexisting well bores, the pipe must be literally forced or “injected” into the well through a sliding seal to overcome the pressure of fluid within the well, until the weight of the pipe in the well exceeds the force produced by the pressure acting against the cross-sectional area of the pipe. However, once the weight of the pipe overcomes the pressure, it must be supported by the injector. The process is reversed as the pipe is removed from the well.
Coiled tubing is faster to run into and out of a well bore than conventional jointed or straight pipe and has traditionally been used primarily for circulating fluids into the well and other work over operations, rather than drilling. However, coiled tubing has been increasingly used to drill well bores. For drilling, a turbine motor is suspended at the end of the tubing and is driven by mud or drilling fluid pumped down the tubing. Coiled tubing has also been used as permanent tubing in production wells. These new uses of coiled tubing have been made possible by larger diameters and stronger pipe.
When in use, a coiled tubing injector is normally mounted to an elevated platform above a wellhead or is mounted directly on top of a wellhead. A typical coiled tubing injector is comprised of two continuous chains, though more than two can be used. The chains are mounted on sprockets to form elongated loops that counter rotate. A drive system applies torque to the sprockets to cause them to rotate. In most injectors, chains are arranged in opposing pairs, with the pipe being held between the chains. Grippers carried by each chain come together on opposite sides of the tubing and are pressed against the tubing. The grippers, when they are in position to engage the tubing, ride or roll along a skate, which is typically formed of a long, straight and rigid beam. The injector thereby continuously grips a length of the tubing as it is being moved in and out of the well bore. Each skate forces grippers against the tubing with a force or pressure that is referred to as a normal force, as it is being applied normal to the surface of the pipe. The amount of traction between the grippers and the tubing is determined, at least in part, by the amount of this force. In order to control the amount of the normal force, skates for opposing chains are typically pulled toward each other by hydraulic pistons or a similar mechanism to force the gripper elements against the tubing. However, the skates could also be pushed. Examples of coiled tubing injectors include those shown and described in U.S. Pat. Nos. 5,309,990, 6,059,029, and 6,173,769, all of which are incorporated herein by reference.
A drive system for a coiled tubing injector includes at least one motor. For larger injectors, intended to carry heavy loads, each chain will typically be driven by a separate motor. The motors are typically hydraulic, but electric motors can also be used. Each motor is coupled either directly to a drive sprocket on which a chain is mounted, or through a transmission to one or more drive sockets. Low speed, high torque motors are often the preferred choice for injectors that will be carrying heavy loads, for example long pipe strings or large diameter pipe. However, high speed, low torque motors coupled to drive sprockets through reduction gearing are also used.
If only one motor is used, it can be used to drive one of the two chains, with the other chain not being driven, or it can be coupled to both chains through a gear or gear train. If separate motors are used to drive each chain, each is coupled to a chain independently of the other. In such arrangements, the chains can be synchronized using a timing gear to cause precise rotational coordination of the two drive sprockets. Such systems are designed so that each drive sprocket turns at exactly the same rotational speed, thereby causing the injector chains to move at the same speed relative to one another, in terms of number of chain links per time.
However, if each chain link is not precisely the same length, and they are not likely to be, then the chains are moving at different speeds relative to each other in terms of distance per time, and one of the chains must then slip with respect to the pipe. The traction of the grippers on the pipe is proportional to the normal force that the skate system applies to the grippers in contact with the pipe. If the normal force is so high as to prevent the slipping, the longer chain will tend to bunch at the slack side entering the grip zone, which is the area between the chains. Chain bunching can cause damage to the chain, the grippers and/or the pipe. To avoid bunching, the normal force must be carefully controlled to allow the chains to slip with respect to the tubing as the difference in length accumulates. However, not enough force can result in out-of-control slipping of the tubing into the well bore, creating substantial damage. Thus, when choosing a normal force, an operator of the injector is forced to carefully balance beneficial slipping that controls the change in length accumulation with the risk of an out-of-control slip of the tubing through the injector.
Because injector chains are inherently timed or synchronized by being in contact with the opposing sides of the same tubing, the choice is often made to forgo the benefits of precisely controlled synchronization. In an unsynchronized injector, each chain is driven independently, which permits each chain to rotate at different speeds. With such a system, minor differences between the length of the chains are not an issue, since the drives can rotate at different speeds to accommodate the differences in chain length without causing slipping. This produces a smooth and efficient drive system.